The way Li’s team did it was to avoid embryos, and instead edit “spermatogonial” stem cells growing in his lab. These are the factory cells that make sperm. By gene editing mouse sperm cells first, and then using corrected sperm to make embryos, Li’s mice came out perfect every time.

Li, who is part of the large Chinese delegation travelling to Washington, says he is convinced that embryo editing “is unacceptable” and that correcting sperm cells is “the only possible strategy.”

Of course, edited sperm would only help prevent genetic diseases passed along by the father. Yet some scientists agree that sperm may be the most practical avenue for making designer people. “It could turn out to be a very good option,” says George Church, a professor at Harvard University and one of the inventors of the CRISPR technology.

Any gene-edited baby would likely start life in an IVF clinic where doctors could oversee its creation. The problem is that rewriting a gene with CRISPR is still hit or miss—the edits only work about 20 percent of the time, and occasionally it introduces unwanted mistakes. That’s no obstacle for engineering flies or worms. Just try again. But it’s a huge problem for anyone who wants to repair an embryo in an IVF clinic and then make a baby.

Editing sperm stem cells overcomes this problem, says Li. He says his team simply first identified stem cells that had been correctly edited and used those to produce swimming sperm, and after that healthy mice. He was also able to check that no accidental edits, or “off target” effects, had occurred.

There are still obstacles to making Li’s sperm technique work in humans. The biggest: no one has figured out how to very successfully cultivate human sperm cells in the lab. “That’s the bottleneck,” says Kyle Orwig, a scientist at the University of Pittsburgh who specializes in male reproduction. “We have to learn how to culture them.”

Scientists say working with eggs will be harder. Men produce millions of new sperm a day—clear proof of stem cells at work. But women’s bodies work differently. Women appear to be born with all the eggs they’ll ever have, and biologically, it seems unlikely there is such a thing as an egg stem cell in adults.

Yet other scientists are already developing a work-around. They predict it will eventually be possible to take a skin cell from a woman and use a technology called “reprogramming” to convert it into an artificial egg in the laboratory. And they could make sperm the same way.

That means IVF clinics in the future might take a skin punch from a customer and return either gene-edited eggs or sperm a few weeks later. “There is no reason to think it can’t be done,” says George Daley, a noted stem cell researcher at Harvard University’s medical school. From what he’s seen, he says, the prospect of installing custom DNA edits into lab-grown reproductive cells is “very real.”